An image forming apparatus which develops an electrostatic image formed on a photosensitive drum by adhering toner and transfers the developed image onto a transfer paper wound around a transfer drum is known.
Such an image forming apparatus includes, for example, two corona chargers within a cylinder 501 having a dielectric layer 501a as shown in FIG. 69: one is a corona charger 502 for attracting a transfer paper P, and the other is a corona charger 504 for transferring a toner image formed on the surface of a photosensitive drum 503 onto the transfer paper P. Including two corona chargers 502.cndot.504 makes it possible to attract the transfer paper P and transfer the toner image onto the transfer paper P independently.
Another image forming apparatus shown in FIG. 70 includes a two-layer structure cylinder 601 made of a semi-conductive layer 601a serving as an outer layer and a base material 601b serving as an inner layer, and a grip mechanism 602 for holding the transported transfer paper P around the cylinder 601. This image forming apparatus grips the end of the transported transfer paper P to hold the same around the surface of the cylinder 601 by means of the grip mechanism 602 first, then charges the surface of the cylinder 601 with electricity either by applying a voltage to the semi-conductive layer 601a serving as the outer layer of the cylinder 601 or triggering a discharge of a charger installed within the cylinder 601, and then transfers a toner image formed on the photosensitive drum 503 onto the transfer paper P.
However, the cylinder 501 of the image forming apparatus shown in FIG. 69 must have two corona charges 502.cndot.504 inside thereof, because the cylinder 501, which serves as a transfer roller, is of a single layer structure using the dielectric layer 501a alone. This structure limits the size of the cylinder 501 and presents a problem that the image forming apparatus can not be downsized.
In contrast, the cylinder 601 in the image forming apparatus shown in FIG. 70, which serves as the transfer roller, is charged by exploiting its two-layer structure to transfer the toner image onto the transfer paper P, and thus the number of the chargers can be reduced. However, the grip mechanism 602 complicates the entire structure of the image forming apparatus. Moreover, the semi-conductive layer 601a serving as the outer layer and the base material 601b serving as the inner layer must be fixed with mounting hardware and secured to each other by small screws, a double-sided adhesive tape or the like to assemble the cylinder 601. Accordingly, the image forming apparatus requires more components and presents a problem that the manufacturing costs increase.
To eliminate these problems, Japanese Laid-open Patent Application No. 2-74975/1990 discloses an image forming apparatus including a corona charger driven by a unipolar power source in the vicinity of a point where a transfer paper separates from a transfer drum made of a lamination of conductive rubber and a dielectric film on a grounded roll of metal.
With this image forming apparatus, a transfer paper is attracted to the transfer drum by inducing the charges on the dielectric film by means of the corona charger. Once the transfer paper is attracted, more charges are induced on the dielectric film, thereby enabling the transfer of a toner image onto the transfer paper.
Since this image forming apparatus uses a single charger to charge the surface of the transfer drum so as to attract the transfer paper and transfer the toner image onto the transfer paper, the transfer drum can be downsized. Also, the above image forming apparatus omits a mechanism such as the grip mechanism 602, so that the transfer paper can be attracted to the transfer drum by a simple structure.
However, since the transfer paper adheres to the transfer drum electrostatically in this image forming apparatus, some charges remain on the transfer drum, which may cause the toner to adhere to the surface of the transfer drum. Thus, these residual charges present problems such as insufficient adhesion of the transfer paper to the transfer drum or back transfer on the transfer paper, thereby degrading the quality of a resulting image.
Accordingly, Japanese Laid-open Patent Application No. 6-51645/1994 discloses a transfer device provided in the vicinity of the transfer drum in an image forming apparatus, which includes cleaning means made of a conductive fur brush for scraping off the toner adhering to the transfer drum and charge removing means for removing the charges caused by the friction between the conductive fur brush and transfer drum. Note that the charge removing means applies a voltage to the conductive fur brush in a polarity reversed to that of the surface potential of the transfer drum, so that the residual charges on the transfer drum are removed. Since not only the charges remaining on the transfer drum are removed, but also the transfer drum is cleaned, the transfer paper can adhere to the transfer drum satisfactorily and the back transfer on the transfer paper can be eliminated, thereby making it possible to produce a good-quality image.
Also, Japanese Laid-open Patent Application No. 3-102385/1991 discloses a cleaning device for an image forming apparatus which attracts a transfer paper to the surface of the transfer drum electrostatically. The cleaning device removes post-transfer residual toner on the surface of a transferring body by applying a bias voltage to a brush cleaner in a polarity reversed to that of the toner. As shown in FIG. 71, the cleaning device includes a conductive brush 702 which makes contact with the inner side of a transfer drum 701 and a cleaning brush 703 which makes contact with the outer surface of the transfer drum 701. According to this structure, the charges remaining on the transfer drum 701 are removed by the conductive brush 702, while the surface of the transfer drum 701 is cleaned by the cleaning brush 703. Thus, the transfer drum can attract the transfer paper satisfactorily and the back transfer on the transfer paper can be eliminated, thereby making it possible to produce a high-quality image.
However, the image forming apparatus disclosed in Japanese Laid-open Patent Application No. 2-74975/1990 charges the surface of the transfer drum through an atmospheric discharge by a corona charger. For this reason, if a color image is formed by repeating a transfer process a number of times, the charges are replenished by the corona charger each time a toner image is transferred onto the transfer paper. Thus, the image forming apparatus demands a charging unit comprising a unipolar power source or the like to drive the corona charger under its control. As a result, the number of components of the image forming apparatus increases, thereby presenting a problem that the manufacturing costs increase.
In addition, a flaw on the surface of the transfer drum makes an electric field area developed by the atmospheric discharge smaller, and the electric field becomes out of balance over the flaw. Such off-balance of the electric field causes a defect in a transferred image such as a white spot (void), and hence degrades the quality of a resulting image.
Also, a considerably high voltage must be applied to charge the surface of the transfer roller through the atmospheric discharge, and the driving energy of the image forming apparatus increases accordingly. Further, since the atmospheric discharge is susceptible to the environments such as the temperature and humidity of air, the surface potential of the transfer roller varies easily, which causes insufficient adhesion of the transfer paper, disordered printing, etc.
The transfer device in the image forming apparatus disclosed in Japanese Laid-open Patent Application No. 6-51645/1994 and the cleaning device disclosed in Japanese Laid-open Patent Application No. 3-102385/1991 remove the residual toner and charges on the surface of the transferring body (transfer drum) by bringing the cleaning brush into contact with the surface of the transferring body. Thus, the cleaning brush may cause a flaw on the surface of the transferring body, and the flaw on the transferring body causes a defect in the transferred toner image and degrades the quality of a resulting image.
Further, the transfer device in the image forming apparatus disclosed in Japanese Laid-open Patent Application No. 6-51645/1994 employs the conductive fur brush to prevent the transfer drum from being charged with electricity caused by the friction between the transfer drum and the brush portion while the transfer drum is being cleaned, and to remove the charges on the transfer drum. The charges on the transfer drum are removed by applying a voltage to the fur brush in a polarity reversed to that of the surface potential of the transfer drum. However, a structure such that enables satisfactory charge removal is not fully concerned, and the removal of the surface potential is not ensured in this application. Thus, there still occur problems that the residual toner causes a smudge on the back of the transfer paper and the residual charges cause insufficient adhesion of the transfer paper to the transfer drum.
Japanese Laid-open Patent Application No. 5-173435/1993 discloses an image forming apparatus which includes a transfer drum having at least an elastic layer made of a foam body and a dielectric layer covering the elastic layer. This image forming apparatus produces a color image on a transfer sheet by sequentially forming a plurality of toner images in their respective colors on a photosensitive drum and superimposing the toner images sequentially on the transfer sheet.
The above image forming apparatus applies a voltage to an attracting roller serving as charge giving means as a technique to hold the transfer sheet on the transfer drum, so that the transfer drum attracts the transfer sheet electrostatically. A space is formed between the elastic layer and dielectric layer to enhance an adhesion force, or namely, the adhesion of the transfer sheet to the transfer drum.
The image forming apparatus disclosed in Japanese Laid-open Patent Application No. 5-173435/1993 specifies neither the hardness of the elastic layer (foam body layer) nor the contacting pressure between the attracting roller and transfer drum. Further, the application is silent about the width of a close contacting portion between the attracting roller furnished with a power source and transfer drum (known as the nip width), and the time required for an arbitrary point on the transfer sheet to pass by the nip width (known as the nip time). Thus, the nip time is assumed to be constant regardless of the kind of the transfer sheet.
However, it is known that the amount of charges given to the transfer sheet during a constant nip time varies depending on the kind of the transfer sheet. Thus, it is assumed that, when the transfer drum attracts the transfer sheet electrostatically, the electrostatic adhesion force differs considerably depending on the kind of the transfer sheet. That is to say, given a constant nip time to all kinds of the transfer sheets, some kinds of the transfer sheets may not adhere to the transfer drum electrostatically in a satisfactory manner, because the amount of the charges given to the transfer sheet during the constant time varies considerably depending on the kind of the transfer sheet. Therefore, as the electrostatic adhesion force decreases over time, there may be a case that the transfer sheet separates from the transfer drum before all of the toner images in their respective colors formed on the photosensitive drum are transferred onto the transfer sheet, thereby presenting a problem that the toner images are not transferred satisfactorily.
Further, the above image forming apparatus demands at least two power sources: an attracting roller's power source for enabling the transfer drum to attract the transfer sheet, and a power source for applying a voltage to the transfer sheet in a polarity reversed to that of the toner when transferring a toner image onto the transfer sheet. Accordingly, there occurs a problem that the manufacturing costs increase.
In addition, Japanese Laid-open Patent Application No. 4-256977 discloses an image forming apparatus including an attracting roller for giving charges to transfer means to enable the transfer means to attract a transfer paper, and attracting voltage applying means for applying an attracting voltage to the attracting roller.
Also, Japanese Laid-open Patent Application No. 4-256978 discloses an image forming apparatus including, in addition to the above-mentioned attracting roller and attracting voltage applying means, transferring voltage applying means for applying a voltage to the transfer means to enable the transfer means to transfer a toner image onto the transfer paper.
In the image forming apparatuses disclosed in the above Japanese Laid-open Patent Application Nos. 4-256977 and 4-256978, the transfer paper is attracted to the transfer means in a reliable manner, and thus the toner image is transferred onto the transfer paper satisfactorily, thereby making it possible to produce a high-quality image.
However, the above two image forming apparatuses apply a high voltage to the attracting roller in the same polarity as that of the voltage applied to the transfer means. Thus, both the image forming apparatuses demand a high voltage power source, or namely, an attracting bias power source, which not only increases the number of components but also demands a safeguard against the high voltage, such as measures for leakage and insulation. Accordingly, the resulting image forming apparatuses becomes more expensive and has more complicated structure.